THE Earth may be under bombardment from ultradense nuggets of dark matter. But the mountain of evidence from seismic monitoring stations that could shed light on this intriguing idea is being routinely thrown away.
Seismic activity, the signals from low-frequency sound waves travelling through the Earth鈥檚 crust, is routinely monitored by a global network of around 5000 government-sponsored sites. Researchers at these monitoring stations are only interested in signals that emanate from a fixed point, such as an earthquake or an illegal underground nuclear test. Geological surveys regularly publish so-called 鈥渃lean鈥� catalogues of seismic activity that appears to originate from such sources. Other signals are written off as noise鈥攑erhaps caused by something as trivial as a passing lorry鈥攁nd discarded.
But this filtering process could be throwing away vital evidence of much more bizarre events. In 1984, Ed Witten of the Institute for Advanced Study in Princeton, New Jersey, suggested that some of the Universe鈥檚 mysterious dark matter, currently detected only by its gravitational effect, could be in the form of ultradense grains called strange quark nuggets. In 1995, Eugene Herrin and colleagues at Southern Methodist University in Dallas, Texas, worked out what kind of seismic activity such a nugget would produce if it passed through the Earth. 鈥淚t would be 10 micrometres across, weigh about a tonne and be going 400 kilometres per hour,鈥� he says. 鈥淚t should radiate sound waves all along its path.鈥�
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Herrin has now obtained raw data from the US Geological Survey and the Australian Seismological Network between 1990 and 1993. In work submitted to the Journal of Seismic Exploration he has found two signals that are consistent with what he calls a 鈥渓inear disturbance鈥� that could be caused by strange quark nuggets rattling through the Earth. The two linear quakes occur in different places and different directions, but both move at about the predicted speed of a strange quark nugget.
So far, astrophysicists are undecided about Herrin鈥檚 claim, because there鈥檚 no other evidence that the nuggets exist. 鈥淭he existence of stable quark nuggets is conceivable, but it鈥檚 a long shot,鈥� says Witten.
But seismologists are taking the claim more seriously. 鈥淚f this paper stands up to criticism, it is an important result,鈥� says Paul Richards of Columbia University in New York City. Even if the linear disturbances Herrin and colleagues have seen aren鈥檛 strange quark nuggets, they鈥檙e definite signals that need an explanation. 鈥淭here are thousands of seismic stations out there assuming all they鈥檙e going to see are point sources,鈥� says Herrin. 鈥淲e鈥檙e asking people to look again.鈥�